1. Field of the Invention
The present invention relates to an image reading apparatus, and more in particular to an image reading apparatus and its driving method that brings a detecting object into contact with a sensor array having a plurality of sensors arranged in a matrix form to detect a contact state of a specific detecting object such as a human body and the like and to execute a reading operation of an image pattern of the detecting object.
2. Description of the Related Art
As a two-dimensional image reading apparatus that reads a printed material, a photograph, or a fine concave and convex shape such as a fingerprint, there is used, for example, a structure that places a detecting object on a detecting surface formed on a photosensor array having photoelectric transducers (photosensors) arranged in a matrix form to be brought into contact therewith to read an image pattern of the detecting object.
In the image reading apparatus having such the structure in which the detecting object directly comes in contact with the detecting surface, there is known one that has a function (hereinafter referred to as “contact detecting function”) of detecting a contact state of the detecting object with the detecting surface to start the image reading in order to perform an appropriate image reading operation as suppressing deterioration in device characteristics of the photosensors. Moreover, there is known one that has a function (hereinafter referred to as “electrostatic removing function”) of discharging and removing static electricity in order to suppress device damage by static electricity charged onto the detecting object and generation of an erroneous operation.
A brief explanation will be hereinafter given of the conventional structure of the image reading apparatus having the aforementioned contact detecting function and the electrostatic removing function with reference to the drawings. In addition, a fingerprint reading apparatus will be hereinafter explained as the structural example of the image reading apparatus.
First, the conventional contact detecting function will be explained.
FIG. 25 is a schematic structural view illustrating one structural example of the conventional contact detecting function, and FIG. 26 is a schematic structural view illustrating another structural example. The contact detecting function illustrated in FIG. 25 is one that is called a resistance detecting system.
Schematically, this system is structured to have a photosensor array 300A having a plurality of photosensors 310 arranged on one surface side of a transparent insulating substrate in a matrix form, transparent electrode layers 320x and 320y formed on an array area where at least the plurality of photosensors 310 is arranged to divide the array area into two to be spaced therebetween via a slight gap GP, a detecting circuit 330a, which applies a D.C. voltage to either one of the transparent electrode layers 320x and 320y (for example, transparent electrode layer 320x) through a lead wire PLx and applies a ground potential to the other transparent electrode layers (for example, transparent electrode layer 320y) through a lead wire Ply to detect a change in voltage when a detecting object such as a finger FG is placed between the transparent electrode layers 320x and 320y to be brought into contact therewith to start the image reading operation in the image reading apparatus, and a surface light source (not illustrated) arranged on a back surface side of the photosensor array 300A.
In such the image reading apparatus, when the detecting object such as the finger FG is placed to be laid across the transparent electrode layers 320x and 320y and is brought into contact therewith, the detecting circuit 330a observes a change in voltage generated when an electrical conduction between the transparent electrode layers 320x and 320y is made through the electric resistance of the finger FG, thereby detecting that the finger is placed on the photosensor array 100p to operate various kinds of drivers and the surface light source (not illustrated), and to automatically execute the image reading operation of the image pattern (fingerprint) of the detecting object.
Moreover, the contact detecting function illustrated in FIG. 26 is one that is called a capacitance detecting system.
Schematically, this system is structured to have a photosensor array 300B having a plurality of photosensors 310 arranged in a matrix form, a transparent electrode layer 320z formed to cover the entirety of the array area, a detecting circuit 330b, which is connected to the transparent electrode layer 320z through a lead wire PLz to detect a change in capacitance when a detecting object is placed on the transparent electrode layer 320z to be brought into contact therewith to start the image reading operation in the image reading apparatus, and a surface light source (not illustrated) arranged on a back surface side of the photosensor array 300B.
In such the image reading apparatus, when the detecting object such as the fingerprint FG is placed on the transparent electrode layer 320z and brought into contact therewith, the change in the capacitance generated when the finger (human body) as a dielectric is contacted and added in connection with the capacitance, which the photosensor array 300B as such originally has, is observed, thereby detecting that the finger is placed on the photosensor array 300B to automatically execute the mage reading operation of reading the fingerprint.
An explanation will be next given of the conventional electrostatic removing function.
FIG. 27A is a schematic structural view illustrating one structural example of the conventional electrostatic removing function in the image reading apparatus.
Schematically, this system is structured to have a photosensor array 300C having a plurality of photosensors 310 arranged on one surface side of a transparent insulating substrate in a matrix form, a transparent electrode layer 330z formed to cover the entirety of the array area where at least a plurality of photosensors 310 is arranged, a lead wire PLp, which connects the transparent electrode layer 330z to a ground potential, and a surface light source (not illustrated) arranged on a back surface side of the photosensor array 300C. Additionally, in the figure, Rp is a wiring resistance for the lead wire PLp.
In such the image reading apparatus, when the detecting object such as the fingerprint FG is placed on the transparent electrode layer 330z and brought into contact therewith, electrical charges (static electricity) carried on the finger FG (human body) are discharged to the ground potential through the lead wire PLp. Namely, since overcurrent, which is caused by the electrical charges carried on the finger FG, flows into the ground potential through the lead wire PLp (wiring resistance Rp), which is the relatively low resistance, it is possible to suppress device damage of the photosensors due to static electricity and generation of the erroneous operation of the image reading apparatus. Here, in conventional, it is known that discharge voltage generated by contact of the figure is generally 3 to 4 kV, so that it is considered that electrostatic withstand pressure may be 5 kV or more. Then, in order to obtain this electrostatic withstand pressure, sheet resistance for the transparent electrode layer 330z was set to a value lower than about 50 Ω/□ and preferably about 15 to 20 Ω/□.
Moreover, the image reading apparatus having both the contact detecting function and electrostatic removing function is also known. FIG. 27B is a schematic structural view illustrating one structural example of a case in which the image reading apparatus has both the contact detecting function and the electrostatic removing function.
In this case, the transparent electrode layer 330z formed on the photosensor array area is connected to a detecting circuit 330b through the lead wring PLp, and for example, an anti-parallel diode circuit 340z having a pair of diodes connected in parallel to be opposite to each other is connected between the lead wire PLp and the ground potential, and overcurrent, which is caused by the electrical charges carried on the finger FG, flows into the ground potential through the lead wire PLp with wiring resistance Rp and the diode of the anti-parallel diode circuit 340z. 
However, in the aforementioned conventional image reading apparatuses, the following problems were present.
In the image reading apparatus (fingerprint reading apparatus) of the resistance detecting system as illustrated in FIG. 25, there is used the method in which the contact state of the detecting object is detected based on a resistance value obtained when the detecting object comes in contact with both the transparent electrode layers 320x and 320y spaced therebetween through the gap GP, however, when the relevant detecting object is a human body, a resistance value peculiar to the detecting object (human body) largely varies due to influences of an individual difference such as the person's constitution and condition and the like or external environments such as temperature, moisture, and the like. This causes a problem in which the contact state of the detecting object cannot be correctly detected and control of starting the image reading operation became ununiform and unstable.
While, in the image reading apparatus of the capacitance detecting system as illustrated in FIG. 26, as one of the methods for correctly detecting the contact state of the detecting object, there is used the method of reading a change in a weak signal voltage that varies in accordance with a capacitance component which the detecting object has, however, in order to judge the change in such a weak voltage, it is desirable that not only the capacitance of the transparent electrode layer but also a parasitic capacitance, which is generated between the photosensitive sensor and the transparent electrode layer, should be extremely small. However, it is necessary to form the transparent electrode layer relatively thick such that the transparent electrode layer has a sufficiently small sheet resistance in order to improve electrostatic withstand pressure of the photosensors and the peripheral circuits. Here, when general metallic oxide is used as a transparent electrode layer, this has a characteristic of a relatively high electrical resistivity, so that when the transparent electrode layer is deposited thick to reduce the sheet resistance as mentioned above, the capacitance of the transparent electrode layer as such largely increases and the parasitic capacitance between the photosensor and the transparent electrode layer increases to reduce a signal to noise ratio (S/N) with respect to the change in the capacitance due to contact of the detecting object, and this causes a problem in which there is difficulty in detecting the change in the capacitance satisfactorily when the detecting object (human body) is placed on the detecting surface.
Moreover, the aforementioned contact detecting system and capacitance detecting system pay attention to only the electric resistance value of the detecting object or the capacitance value to detect the change generated thereby, and this causes a problem in which when a foreign object or objects besides a normal object as the detecting object are contacted, there is difficulty in judgment of whether this is a normal detecting object or not.
Moreover, in the image reading apparatus having the electrostatic removing function as illustrated in FIGS. 27A and 27B, since the film material of the transparent electrode layer 330C needs optical transparency and conductivity for discharging static electricity through the lead wire PLp, a transparent conductive film such as a stannic oxide (SnO2) film, an ITO (Indium-Tin-Oxide) film and the like is generally used.
As mentioned above, conventionally, when the sheet resistance of the transparent electrode layer is set to a value lower than about 50 Ω/□ and preferably about 15 to 20 Ω/□, a predetermined electrostatic withstand pressure can be obtained, and it is known that such a value can be obtained by setting a film thickness to approximately about 1500 to 2000 Å (150 to 200 nm) when the ITO film is used as the transparent electrode layer.
By the way, the condition of the sheet resistance value of the aforementioned transparent electrode layer was obtained based on such a condition that the electrostatic withstand pressure was 5 kV or more with respect to the discharge voltage due to contact of the finger as mentioned above. However, the study zealously made by the inventors of the present invention later revealed that the human body was electrically charged to 10 kV in some cases. Then, as the electrostatic withstand pressure against this, it was shown that there was need of a value higher than 10 kV, more specifically a value of 10 to 15 kV.
In contrast to this, it can be expected that necessary electrostatic withstand pressure can be obtained by further setting the transparent electrode layer to have low resistance based on the concept of the prior art, however, in this case, the film thickness of the transparent electrode layer must be much thicker. However, since this transparent electrode layer must have a good transmittance and should not prevent the object image pattern from being read, the film thickness cannot be increased unnecessarily. Moreover, when the capacitance detecting system, as the contact detecting function, using the transparent electrode layer is applied, the increase in the thickness of the transparent electrode layer increases the parasitic capacitance between the photosensor and the transparent electrode layer, and this causes a problem in which there is difficulty in detecting the change in the capacitance satisfactorily when the detecting object (human body) is placed on the detecting surface as mentioned above.